Flex crack resistance is an extremely important property for film used in bags and pouches that are formed into packages for flowable materials, particularly for liquids, and most particularly for non-viscous liquids like water, milk, juices, and the like. These liquids can slosh around considerably during package manufacturing, handling and transportation causing flexing of the film and flex cracking for most of the commonly used film materials.
Flex cracking is caused by the movement of the liquid within the pouch or bag, and is most likely to happen where the film is in close proximity to the upper surface of the liquid. Flex cracking can occur during shipping and handling of even the smallest fluid-containing pouches. Flex crack pinholes result in at least loss of oxygen and moisture barrier, reducing the shelf life potential of the product, and often in loss of the hermetic seal, rendering the product unsafe to use if it is a food product. Generally a Flex Crack Resistant Film is one that should develop 10 or less pinholes per 300 cm2 in 20,000 cycles of Gelbo Flex testing, and preferably 5 or less pinholes per 300 cm2 in 20,000 cycles.
It is well known that film made from a lower density polyethylene will have better flex crack resistance than film made from a higher density polyethylene. In this regard, reference may be had to the disclosures of WO 95/26268. It is also well known that film made from a lower density polyethylene will have inferior thermal resistance and stiffness than film made from a higher density polyethylene. Reference in this instance may be had to the disclosures of US 2005/0131160 published Jun. 16, 2005, the disclosure of which are incorporated herein by reference. However, what is not well understood is how to modify the composition of a polyethylene film to maximize the improvement in flex crack pinhole resistance, while at the same time minimizing the negative effect on thermal resistance and stiffness, which are generally desirable film properties.
Film with inadequate thermal resistance may stretch and deform unacceptably in close proximity to heated machine parts such as sealing jaws found in form, fill and seal machines. The stretched or deformed area of the film may become the weak point of the pouch or bag, at which it will fail prematurely in subsequent shipping and handling. Aqueous products are a major portion of those products packaged in pouches and bags. As water boils at 100° C., thermal stability at temperatures just above 100° C. is therefore desirable in a pouch or bag film.
Bending stiffness may, or may not, be important to the performance of the pouch or bag, depending on the end use. Pouches, which are to stand up in a pitcher and pour without flopping over to restrict fluid flow, need a minimum level of bending stiffness. Film also requires a certain amount of bending stiffness to run through form-fill-seal equipment effectively, without conforming too closely around forming collars, stationary guides and rollers so that it stretches and distorts. Bending stiffness depends on the thickness of the film and its tensile modulus. As economics drive industry to downgauge films further and further, tensile modulus becomes more and more important in achieving adequate bending stiffness. The minimum tensile modulus for a thin pouch or bag film that is used on form-fill-seal equipment should be 20,000 psi, and 25,000 psi may also be used.